The invention relates to methods and a device for fast creation of fluid interfaces and use of this device for the determination of liquid-liquid and liquid-gas interfacial properties.
The value of the equilibrium or dynamic ST/IFT of liquids determines processes like wetting, spreading, foaming, emulsification, coalescence, etc. The adsorption to a surface and the rate of adsorption directly affect the dynamic surface tension and are especially important for foaming and emulsification. The rate of adsorption of the surfactants on newly formed surfaces and interfaces depends on several factors among which are: type and bulk concentration of the surfactant, electrolyte concentration, temperature, diffusivity, hydrodynamic conditions, etc.
Many experimental methods for measurement of the equilibrium and dynamic ST/IFT exist. More recent reviews can be found in several books [1-4]. These methods differ by (i) the procedure of formation of the interface as well as (ii) by the technique of determination of the ST/IFT. The following sections shortly describe the procedures and the techniques and show drawbacks of the currently known methods and devices and their improvements by the claims applied for in this application:
(i) Formation of Interface
In principle, every method of formation of a new interface can be used for measuring both equilibrium and dynamic ST/IFT, provided that the time for surface formation can be regulated to be much shorter than the characteristic time of adsorption. The most widely used methods, Wilhelmy plate, du Noüy ring, Pendant and Sessile drops in their regular application do not meet this requirement and can be used for studying only very slow kinetics of adsorption. The methods based on drop or bubble formation by pushing a liquid or gas through a fluid to create a fresh interface are much faster. The most widely used methods of this kind are the Bubble Pressure Method (BPM) and the Drop Volume Method (DVM). In both methods the area and the shape of the interface change significantly during the measurement, which creates problems with the data interpretation and decreases the precision of calculation of the ST/IFT. In the methods described in claims 1 to 3 of this patent, the drop/bubble is formed for very short time and practically does not change its size during the measurement.
(ii) Determination of the ST/IFT
There are two main techniques for obtaining data and calculating the IFT/ST with drops and bubbles. The first technique, known as “Axisymmetric Drop Shape Analysis” (ADSA), uses the acquired shape of the drop/bubble surface. This technique is applicable only to deformed drops or deformed bubbles. The main idea of the method is that the shape of any fluid surface/interface under gravity is generally governed by the balance between the gravity and the capillary force. Under equilibrium conditions the mathematical description is given by the Young-Laplace equation:ΔP=σ(1/R1+1/R2),where σ is the interfacial tension, R1 and R2 are the principle radii of curvature and ΔP is the pressure difference across the drop interface. If gravity is the only external force then ΔP is given byΔP=ΔP0+Δρgz, where ΔP0 is the pressure difference at a reference plane (usually it is the plane passing through the apex of the drop/bubble), Δρ is the density difference between the fluids in the inner phase and the surrounding media, g is the acceleration due to gravity and z is the vertical coordinate of the drop/bubble generatrix, measured from the reference plane. The numerical solution of the equation allows obtaining the ST/IFT from the fit of the bubble/drop shape. This method has been widely applied for almost 20 years already [1-10]. There are also some apparatuses commercially available based on it (e.g. DSA10, DSA100, EasyDrop by Krüss, GER; FTA200, FTA2000 by First Ten Angstroms, USA; CAM 100, CAM 200 by KSV, Finland; ODT200, HFDT 500, by TECLIS-I.T.CONCEPT, France; PAT-1 by Sinterface, GER; OCA10, OCA20 by Dataphysics, GER). The ADSA has some requirements restricting its application. The formed drop/bubble must be large and deformed enough (the shape factor, or Bond number must be greater than e.g. 0.4 [9, 10]) in order to obtain the value of the IFT/ST with a satisfying accuracy, e.g. better than 0.1 mN/m. The Bond number is defined asBo=ΔρgR2/σ,where R is the radius at the apex of the drop/bubble. Frequently it is not possible to achieve a high precision because:                large drops especially with a low IFT are not easily kept attached to the capillary tip;        large drops are subject to considerable mechanical vibrations;        no satisfactory drop deformation can be achieved for interfaces between fluids with small density difference.        
The second commonly used technique for determination of the ST/IFT is the Capillary Pressure Method (CPM). This technique is applicable only to spherical drops or bubbles. The method is based on measuring the pressure difference PC across a spherical drop interface with known radius of curvature R. The Young-Laplace equation in the formPc=2σ/R is used to determine the value of the ST/IFT σ. When applying CPM in fact one measures a pressure difference P, which is sum of two components:P=Pc+P0,where P0 comprises all other contributions unrelated to the ST/IFT. The hydrostatic contribution depends on the relative position of the drop/bubble and the membrane of the pressure detector and the liquid density. The measured total pressure signal P often includes other constant additions (offsets of the electronics, etc.), which can be assigned to the term P0. Usually the drop/bubble radius R is determined from digitized images obtained by video camera. The method has been applied for more than 15 years already [11-19] (for reviews see [1, 2]) but mostly in laboratory setups for research purposes. Commercial instruments using this method have appeared in the last two years (EDM/ODM-Module for DSA100, Krüss, GER; DPA-1 for PAT-1, Sinterface, GER). The module produced by Krüss [18], is used for surface rheology characterization along with surface tension measurements.
There are two peculiarities of the CPM which are the main reasons for its rare application so far:                the requirement of a spherical shape of the drop makes the method applicable under gravity for low Bond numbers only (Bo<0.1), which is fulfilled with small drop/bubble radius (e.g. smaller than 0.5 mm), or with systems with a small density difference of the fluids forming the interface [1, 2, 11-19]. That is why instruments based on this method have been constructed for use under microgravity conditions [2].        the measurement by pressure detection relies on complex and time-consuming calibration procedures for the determination of P0, which sometimes leads to lack of accuracy.        